An oil-immersed transformer installed in a transformer substation or an electric power plant is one of the more important components of an electric power supplying system, and it is required to have high reliability. The oil-immersed transformer may have its electrical and mechanical performance degraded due to deterioration during operation, which causes an abnormal condition of the oil-immersed transformer. This phenomenon may cause a serious accident if it may not be detected in advance and appropriately treated.
When an abnormal phenomenon such as dielectric breakdown, local overheating, and the like occurs in the oil-immersed transformer, this phenomenon always accompanies a generation of heat. An insulating material, such as insulating oil, an insulating paper, pressboard, and the like, contacting a heat-generating source is affected by the heat and dissolved by a chemical reaction to generate gases. Most of these gases are dissolved in the insulating oil. Therefore, when gases are extracted and analyzed from the insulating oil taken from the oil-immersed transformer, a type and an extent of the fault occurring in the transformer may be diagnosed.
The type of the internal faults in the oil-immersed transformer and the method of diagnosing the internal fault of the oil-immersed transformer through the dissolved gases are prescribed in the international standard (IEC 60599: Mineral oil-impregnated electrical equipment in service guide to the interpretation of dissolved and free gases analysis and IEEE C57.104: IEEE Guide for the interpretation of gases in oil-immersed transformer), and most electric power companies and users of oil-immersed electric power devices estimate the internal fault depending on the international standard.
The type of fault prescribed in these international standards is classified into an electrical fault and a thermal fault, and particularly, into six faults such as partial discharges, discharges of low energy D1, discharges of high energy D2, a first thermal fault (thermal fault t<300° C.) T1, a second thermal fault (300° C.<thermal fault t<700° C.) T2, and a third thermal fault (thermal fault t>700° C.) T3. Further, gases which are objects to be analyzed in the international standards include five components such as hydrogen H2, methane CH4, ethane C2H2, ethylene C2H4, and acetylene C2H2.
Conventionally, the type of fault is classified by analyzing a composition ratio of the five gases, a content ratio of each of the five gases, a range of a key gas, and the like. However, in the conventional method of diagnosing the internal fault of the oil-immersed transformer, there are problems as follows.
First, in the diagnosis method using the key gas, since the fault is diagnosed by using only the key gas (a maximum gas value), a pattern, a composition, and a variation according to energy in each fault cannot be applied to the diagnosis method, resulting in an increase in a ratio of a wrong diagnosis. For example, if not the key gas but another gas has the maximum value, there is a problem in that there is present a region in which the diagnosis is impossible.
On the other hand, in the diagnosis method using the composition ratio of the five gases, since a result of the diagnosis is established by applying the pattern, the composition, and the variation of the gas to the diagnosis, an accuracy of the diagnosis is higher. However, it is impossible to diagnose the internal fault of the transformer when the internal fault does not fall within the type of the fault and a ratio of gas in each fault.
Meanwhile, in the diagnosis method using the content ratio of the gas, there is no region in which the diagnosis is impossible, and the accuracy of the diagnosis is higher. However, only three gases having high energy of the fault are used, and hydrogen and ethane generated in the low energy fault are not applied to the diagnosis. Accordingly, there is a problem in that it is difficult to diagnose an initial fault.
Therefore, there is required development of a technology to diagnose the internal fault of the oil-immersed transformer, in which reliability of the diagnosis for the internal fault of the oil-immersed transformer can be improved and all kinds of faults can be diagnosed.